Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography.
Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often, a protective cover sheet.
The support (or backing) layer lends support to the plate. The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefin, polyamides, and the like, including polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). The support may be in sheet form or in cylindrical form, such as a sleeve. The sleeve may be formed from single layer or multiple layers of flexible material. Flexible sleeves made of polymeric films are preferred, as they typically are transparent to ultraviolet radiation and thereby accommodate backflash exposure for building a floor in the cylindrical printing element. One widely used support layer is a flexible film of polyethylene terephthalate.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, processing aids, UV absorbers and dyes. As used herein, the term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional relief pattern of cured material. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen et al., U.S. Pat. No. 3,265,765 to Holden et al., U.S. Pat. No. 4,320,188 to Heinz et al., U.S. Pat. No. 4,427,759 to Gruetzmacher et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may also be used. The photocurable layer(s) may be applied directly on the support. In the alternative, the photocurable layer(s) may be applied on top of an adhesion layer and/or resilient under layer.
Photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, “actinic radiation” is radiation that is capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. In the alternative, a negative may be placed directly on the at least one photocurable layer.
In a “digital” or “direct to plate” process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are then ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Thereafter, the at least one photocurable layer with the in situ negative thereon, is subjected to flood-exposure by UV light through the in situ negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Selective exposure to the source of actinic radiation can be achieved using either the analog or digital method. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
Processing steps for forming relief image printing elements typically include the following:                1) Image generation, which may be mask ablation for digital “computer to plate” printing plates or negative production for conventional analog plates;        2) Face exposure through the mask (or negative) to selectively crosslink and cure portions of the photocurable layer not covered by the mask, thereby creating the relief image;        3) Back exposure to create a floor layer in the photocurable layer and establish the depth of relief. It is preferred to face expose the plate before flipping it for back exposure. Doing the back exposure first may result in damaging the black mask during the plate handling, thus contributing to image degradation. Some exposing systems can also run both exposure systems simultaneously, which also preserves the image integrity;        4) Development to remove unexposed photopolymer by solvent (including water) or dry “thermal” development; and        5) If necessary, post exposure and detackification.        
Removable coversheets may be provided to protect the photocurable printing element from damage during transport and handling. Useful cover sheets include flexible polymeric films, e.g., polystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymers, polyamide or polyesters. Polyesters, especially polyethylene terephthalate, are commonly used.
Prior to processing the printing elements, the coversheet(s) are removed and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer solvent (or thermal development) removes the unexposed areas of the photopolymerizable layer, leaving a printing relief that can be used for flexographic printing.
As used herein “back exposure” refers to a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to sensitize the photocurable layer and to establish the depth of relief.
Prior to the brief back exposure step (i.e., brief as compared to the imagewise exposure step), an imagewise exposure is performed utilizing a digitally-imaged mask or a photographic negative mask, which is in contact with the photocurable layer and through which actinic radiation is directed.
The type of radiation used is dependent in part on the type of photoinitiator in the photopolymerizable layer. The digitally-imaged mask or photographic negative prevents the material beneath from being exposed to the actinic radiation and hence those areas covered by the mask do not polymerize, while the areas not covered by the mask are exposed to actinic radiation and polymerize. Any conventional sources of actinic radiation can be used for this exposure step. Examples of suitable visible and UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units, photographic flood lamps, and, more recently, UV light emitting diodes (LEDs).
LEDs are semiconductor devices which use the phenomenon of electroluminescence to generate light. LEDs consist of a semiconducting material doped with impurities to create a p-n junction capable of emitting light as positive holes join with negative electrons when voltage is applied. The wavelength of emitted light is determined by the materials used in the active region of the semiconductor. Typical materials used in semiconductors of LEDs include, for example, elements from Groups (III) and (V) of the periodic table. These semiconductors are referred to as III-V semiconductors and include, for example, GaAs, GaP, GaAsP, AlGaAs, InGaAsP, AlGaInP and InGaN semiconductors. The choice of materials is based on multiple factors including desired wavelength of emission, performance parameters and cost.
It is possible to create LEDs that emit light anywhere from a low of about 100 nm to a high of about 900 nm. Currently, known UV LED light sources emit light at wavelengths between about 300 and about 475 nm, with 365 nm, 390 nm and 395 nm being common peak spectral outputs. When using LED light sources for curing photocurable compositions, the photoinitiator in the photocurable composition is selected to be responsive to the wavelength of light emitted by the LED light source.
LED offer several advantages over other sources of actinic radiation. For example, LEDs are instant on/off sources requiring no warm-up time, which contributes to LED lamps' low energy consumption. LEDs also generate much less heat, with higher energy conversion efficiency, have longer lamp lifetimes, and are essentially monochromatic, emitting a desired wavelength of light which is governed by the choice of semiconductor materials employed in the LED.
Curing devices that typically incorporate LEDs utilize high intensity UV LEDs arranged in an array or an assembly. For example, the UV LEDs may be arranged in a light bar, in which the light bar and the photocurable printing blank move relative to each other (i.e., the light bar travels over the printing or the plate travels under the light bar), in order to cure the entire plate surface, as described, for example, in U.S. Pat. Pub. No. 2012/0266767 to Klein et al., the subject matter of which is herein incorporated by reference in its entirety. Klein et al. describes both printing sleeves and flat printing plates that may be produced by moving a light exposure unit relative to a printing sleeve or planar printing plate and describes a light exposure unit that includes LED arrays and that the light exposure unit can be used to producing printing elements having flat tops and round tops on the same plate using a digital workflow. U.S. Pat. Pub. No. 2013/0242276 to Schadebrodt et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of producing flexographic printing elements including the steps of exposing the printing element to actinic light at a high intensity with a plurality of UV LEDs and then exposing the printing element to actinic light at a lower intensity from a UV radiation source other than UV-LEDs.
In order to perform the imagewise exposure step in a relatively short period of time, it is generally required that the light bar or other UV light assembly has a very high UV output of 1 W/cm2 or greater at the plate surface. However, this approach is problematic because it can generate a lot of heat and the rapid cure of the polymer can cause the surface to contract or “cup.” In addition, plates cured with high intensity UV LEDs can actually print with more gain that the same plate cured under a lower intensity UV LED.